我国秋季降水的年际变化及与热带太平洋海温异常分布的关系

利用我国160个站点58年(1951～2008年)的降水资料、NCEP/NCAR再分析环流资料和Hadley海表温度资料,对我国秋季降水年际变化的特征和可能成因进行了分析。结果显示,秋季降水前两模态分别反映长江流域及以南地区和长江以北的江淮、黄淮、华北、四川盆地北部至河套等地降水的变化,两降水模态的变化都以年际尺度为主,年代际变化特征不明显。就环流形势而言,第一模态的年际变化主要与西太平洋副热带高压的强度及相应的对流层低层菲律宾群岛附近的异常气旋/反气旋联系紧密,第二模态的年际变化则可能受到副热带高压的南北位置和相应的日本岛附近的异常气旋/反气旋的影响。同时,两模态及相应的异常环流还分别与热带东印度洋和热带西太平洋附近的异常垂直运动关系密切,热带地区的异常垂直运动可能通过经圈方向的异常环流影响到东亚地区。此外,两降水模态不仅与热带地区的异常环流关系密切,而且与热带海温异常也存在紧密的联系。与两模态相关联的热带太平洋海温异常显示出不同的分布特征,当热带东太平洋偏暖/冷,西太平洋偏冷/暖时,长江以南地区降水偏多/少。而当热带东太平洋和中太平洋一致偏暖/冷时,长江以北地区降水易偏少/多。两降水模态与热带海温及热带地区异常环流之间的密切关系显示热带太平洋海温异常的不同分布可能通过激发不同的热带地区异常垂直环流形势而对降水产生影响。

热带太平洋和印度洋海温异常对东亚冬季风影响的一个物理机制

本文利用NCEP/NCAR提供的大气环流资料和海表温度异常资料,在分析热带太平洋和印度洋海温异常与冬季大气环流之间关系的基础上提出了一个综合反映热带太平洋和印度洋海温异常的综合指数。分析表明,冬季太平洋和印度洋海温异常指数的值越大(小),东亚冬季风指数的值越大(小),东亚地区将出现异常的南(北)风的响应,东亚冬季风将越弱(强)。应用加热强迫影响热带环流的简单模式研究了热带太平洋印度洋异常海温对东亚冬季风影响的物理机制。结果表明,当冬季热带太平洋和印度洋海温异常指数处于正(负)位相时,西太平洋区域强迫出异常南(北)风。这是使得东亚冬季风偏弱(强)的重要原因之一。冬季热带太平洋和印度洋海温异常对东亚冬季风影响最为显著的关键区是赤道西太平洋。

南半球对流层大气对热带太平洋—印度洋海温异常综合模的响应及其机制解释

利用NOAA SST及NCEP/NCAR再分析资料,研究了热带太平洋—印度洋海温异常综合模和南半球对流层大气之间的遥相关模态并对其进行了机制解释。首先通过相关和合成分析,给出了遥相关的空间模态,结果表明：北半球秋、冬季,在南半球对流层大气存在和热带太平洋—印度洋海温异常综合模密切联系的遥相关作用中心,该中心的分布构成一列明显的从赤道中太平洋出发,最终到达非洲中南部及赤道印度洋的Rossby波列,将赤道太平洋、印度洋与南半球中高纬度大气连接起来,起到了类似＂大气桥＂的作用。而单纯IOD和单纯EN-SO均难以在南半球对流层激发出遥相关波列,进一步证实了两者共同作用的影响。其次,利用行星波能量传播理论对两者之间的遥相关进行了机制分析,发现纬向波数为1~3的大气行星波的能量传播是热带太平洋—印度洋海温异常综合模与南半球对流层大气之间遥相关的一种可能的联系方式。

热带太平洋-印度洋海温异常综合模及其影响

已有研究表明,由于大气瓦克环流和印度尼西亚灌穿流的影响,太平洋和印度洋的海温异常既有紧密关系、又有各自的一些特点,单独考虑太平洋的ENSO模并不能完全说明海温异常对气候的影响.本文从统一综合考虑太平洋和印度洋海温异常(SSTA)的形势及特征出发,提出了太平洋-印度洋海温异常综合模概念;并进一步比较研究了各个异常模态的不同特征,以及它们对中国气候和印度夏季降水的不同影响.为进一步确认资料分析结果,还用大气环流模式进行了数值模拟试验.其结果清楚地表明,必须考虑和研究太平洋-印度洋海温异常综合模及其影响,从而为短期气候预测提供更好科学依据.

两广地区前、后汛期降水异常反位相现象及可能机理

利用1961-2018年中国地面气象台站2400站中广东省和广西壮族自治区(两广地区)134站逐日降水资料、英国Hadley中心逐月海表温度资料、NCEP/NCAR逐月再分析资料,筛选了两广地区前汛期降水正(负)异常、后汛期降水负(正)异常的年份,即前、后汛期降水异常反位相年份。结果表明,近60 a来,两广地区前、后汛期降水反位相现象与热带西太平洋海温异常存在密切联系。前汛期期间,当热带西太平洋海温为负异常时,大气对该冷源的Rossby波响应引起西北太平洋反气旋环流异常,且水汽由热带西太平洋向两广地区输送并辐合,同时两广地区为水汽的湿平流区域、风场异常辐合区域,其上空受异常上升运动控制,这样的环流配置有利于两广前汛期期间降水正异常。热带西太平洋海温负异常可持续至后汛期期间,大气的Rossby波响应所致的西太平洋反气旋较前汛期偏西北,两广地区为水汽异常辐散区域,同时水汽干平流且风场异常辐散,受异常下沉运动控制,不利于两广地区降水的产生。反之亦然。

中国西南秋季旱涝的诊断分析

摘要：为了分析中国西南秋季旱涝情况，利用西南地区东部1961-2012年的降水资料和区域旱涝指数对该地区秋季旱涝等级进行了评定，并运用NCEP／NCAR再分析资料与英国气象局海温资料对典型旱涝年的成因进行了探讨。结果表明，西南地区东部降水呈显著减少趋势，并在1990年左右发生了突变。旱年秋季，低层在南中国海附近为显著的气旋性环流异常，西南地区上空存在显著的下沉运动，涝年则相反。进一步分析发现，当西太平洋海温偏高时，可以激发罗斯贝波环流异常，对应在南中国海附近存在异常的气旋性环流，临近的西南地区东部处于异常的下沉气流控制，容易发生干旱，涝年则相反。此外，热带西太平洋海温于20世纪80年代中后期发生了由冷转暖的突变，这与随后西南地区东部秋季降水的突变密切相关。

Interannual Variability of the Hadley Circulation Associated with Tropical Pacific SST Anomaly

The seasonal and interannual variability of zonal mean Hadley circulation are analyzed, and the important effects of sea surface temperature(SST), especially the tropical Pacific SST, on the meridional circulation are discussed. Following results are obtained: 1) the Hadley circulation presents a single clockwise(anticlockwise) cross-equator circulation in the Northern(Southern) Hemisphere winter,while it is a double-ring-shaped circulation quasi-symmetric about the equator in spring and autumn. The annual mean state just indicates the residual of the Hadley cell in winter and summer. 2) The first mode of interannual anomalies shows a single cell crossing the equator like the climatology in winter and summer but with narrower width. The second mode shows a double ring-shaped cell quasi-symmetric about the equator which is similar to the Hadley cell in spring or autumn. 3) Vertical motion of the Hadley circulation is driven by sea surface temperature(SST) through latent and sensible heat in the tropics, and the interannual anomalies are mainly driven by the SST anomaly(SSTa) in the tropical Pacific. 4) The meridional gradient of SSTa is well consistent with the lower meridional wind of Hadley circulation in the interannual part. For the spatial distribution, the meridional gradient of SSTa in the Pacific plays a major role for the first two modes while the effects of the Indian Ocean and the Atlantic Ocean can be ignored.

ENSO cycle and climate anomaly in China

The inter-annual variability of the tropical Pacific Subsurface Ocean Temperature Anomaly （SOTA） and the associated anomalous atmospheric circulation over the Asian North Pacific during the E1 Nifio-Southern Oscillation （ENSO） were investigated using National Centers for Environmental Prediction/ National Center for Atmospheric Research （NCEP/NCAR） atmospheric reanalysis data and simple ocean data simulation （SODA）. The relationship between the ENSO and the climate of China was revealed. The main results indicated the following： 1） there are two ENSO modes acting on the subsurface tropical Pacific. The first mode is related to the mature phase of ENSO, which mainly appears during winter. The second mode is associated with a transition stage of the ENSO developing or decaying, which mainly occurs during summer; 2） during the mature phase of E1Nifio, the meridionality of the atmosphere in the mid-high latitude increases, the Aleutian low and high pressure ridge over Lake Baikal strengthens, northerly winds prevail in northern China, and precipitation in northern China decreases significantly. The ridge of the Ural High strengthens during the decaying phase of E1 Nifio, as atmospheric circulation is sustained during winter, and the northerly wind anomaly appears in northern China during summer. Due to the ascending branch of the Walker circulation over the western Pacific, the western Pacific Subtropical High becomes weaker, and south-southeasterly winds prevail over southern China. As a result, less rainfall occurs over northern China and more rainfall over the Changjiang River basin and the southwestern and eastern region of Inner Mongolia. The flood disaster that occurred south of Changjiang River can be attributed to this. The La Nifm event causes an opposite, but weaker effect; 3） the ENSO cycle can influence climate anomalies within China via zonal and meridional heat transport. This is known as the ＂atmospheric-bridge＂, where the energy anomaly within the tropical Pacific transfers to the mid-high latitude in the northern Pacific through Hadley cells and Rossby waves, and to the western Pacific-eastern Indian Ocean through Walker circulation. This research also discusses the special air-sea boundary processes during the ENSO events in the tropical Pacific, and indicates that the influence of the subsurface water of the tropical Pacific on the atmospheric circulation may be realized through the sea surface temperature anomalies of the mixed water, which contact the atmosphere and transfer the anomalous heat and moisture to the atmosphere directly. Moreover, the reason for the heavy flood within the Changjiang River during the summer of 1998 is reviewed in this paper.

Relationship Between North Pacific SST Anomalies and the Atmospheric Circulation Anomalies in January 2008

Based on the NCEP (National Centers for Environmental Prediction) data,the relationship between the Sea Surface Temperature Anomalies (SSTAs) in the North Pacific and the atmospheric circulation anomalies in January 2008 is analyzed in this study.The SSTA mode most correlated with the Geopotential Height anomalies (GHAs) in January 2008 in the North Pacific exhibited a basin-wide horseshoe pattern with a warm center in November 2007.This persistent SSTA pattern would induce positive GHAs in the Aleutian Low area and East Asia and the northward extension of the West Pacific Subtropical High in January 2008 by maximum diabatic heating in the atmosphere over the Kuroshio Oyashio Extension (KOE) area,leading to the occurence of the circumpolar trough-ridge wave train anomaly in January 2008.

INTERANNUAL VARIATIONS OF ATMOSPHERIC HEAT SOURCES AND MOISTURE SINKS OVER THE EQUATORIAL PACIFIC AND THEIR RELATIONS TO THE SST ANOMALIES

The interannual variations of atmospheric heat sources and moisture sinks over the Equatorial Pacific and their relations with the SST anomalies are studied using ECMWF reanalysis data from 1979 to 1993. It is found by singular value decomposition (SVD) analysis that the region in the tropical Pacific with high positive correlation between the vertically integrated heat source <Q1> anomaly and the SST anomaly, and between the vertically integrated moisture sink <Q2> anomaly and the SST anomaly, is mainly located in a long and narrow belt to the east of 170 °E between 5 °S and 5 °N. The analysis of the vertical structure of atmospheric heat sources and moisture sinks shows that the interannual variations of Q1, Q2 and SST in the equatorial central and eastern Pacific are strongly and positively correlated in the whole troposphere except the bottom (962.5 hPa) and the top (85 hPa) layers. However, in the western Pacific, the interannual variations of Q1 below 850 hPa is negatively related to the SST. The correlation coefficient at the level 962.5 hPa reaches even –0.59. In other layers the positive correlation between the interannual variations of Q1, Q2 and the SST are weak in the western Pacific.

Occurrences of Wintertime Tropical Cyclones in the Western North Pacific under the Background of Global Warming

Using tropical cyclone （TC） observations over a 58-yr period （1949-2006） from the China Meteorological Administration, the 40-year ECMWF Reanalysis （ERA-40）, NCEP-NCAR reanalysis, and the Hadley Centre sea ice and sea surface temperature （HadISST） datasets, the authors have examined the behaviors of tropical cyclones （TCs） in the western north Pacific （WNP） in boreal winter （November-December-January-February）. The results demonstrate that the occurrences of wintertime TCs, including super typhoons, have decreased over the 58 years. More TCs are found to move westward than northeastward, and the annual total number of parabolic-track-type TCs is found to be decreasing. It is shown that negative sea surface temperature anomalies （SSTAs） related to La Nifia events in the equatorial central Pacific facilitate more TC genesis in the WNP region. Large-scale anomalous cyclonic circulations in the tropical WNP in the lower troposphere are observed to be favorable for cyclogenesis in this area. On the contrary, the positive SSTAs and anomalous anticyclonic circulations that related to E1 Nifio events responsible for fewer TC genesis. Under the background of global warming, the western Pacific subtropical high tends to intensify and to expand more westward in the WNP, and the SSTAs display an increasing trend in the equatorial eastern-central Pacific. These climate trends of both atmospheric circulation and SSTAs affect wintertime TCs, inducing fewer TC occurrences and causing more TCs to move westward.

Processes involved in the second-year warming of the 2015 El Nio event as derived from an intermediate ocean model

The tropical Pacific experienced a sustained warm sea surface condition that started in 2014 and a very strong El Nio event in 2015. One striking feature of this event was the horseshoe-like pattern of positive subsurface thermal anomalies that was sustained in the western-central equatorial Pacific throughout 2014–2015. Observational data and an intermediate ocean model are used to describe the sea surface temperature(SST) evolution during 2014–2015. Emphasis is placed on the processes involved in the 2015 El Nio event and their relationships with SST anomalies, including remote effects associated with the propagation and reflection of oceanic equatorial waves(as indicated in sea level(SL) signals) at the boundaries and local effects of the positive subsurface thermal anomalies. It is demonstrated that the positive subsurface thermal anomaly pattern that was sustained throughout 2014–2015 played an important role in maintaining warm SST anomalies in the equatorial Pacific. Further analyses of the SST budget revealed the dominant processes contributing to SST anomalies during 2014–2015. These analyses provide an improved understanding of the extent to which processes associated with the 2015 El Nio event are consistent with current El Nio and Southern Oscillation theories.